1. Field of the Invention
The invention relates to a device for the fast variation of the wavelength of lasers, provided with a movable mirror arrangement in the optical path of the laser resonator, by means of which the beam can be directed to a grating, the desired wavelength being adjustable by varying the angle of incidence of the light on the grating.
2. Description of the Prior Art
Lasers permitting fast tuning between two wavelengths are being used in a wide variety of applications, e.g. for the remote measurement of pollutants by means of pulsed lasers using the differential absorption technique. The spacing of the laser pulses of the two different wavelengths should range between one and ten milliseconds.
For fast tuning, it is known to use a grating which rotates in the resonator of a pulsed laser at constant angular velocity. In the course of one rotation period, the grating passes through all possible angular positions which determine the emission frequency of the laser. The desired laser frequency can thus be adjusted by triggering the laser exactly at the time when the angular position of the rotating grating is correct. This ensures full liberty to select the laser frequency at each laser pulse. In the case of rapidly pulsed lasers, the frequency of successive pulses can be arbitrarily determined. Instead of a rotating grating, a rotating mirror or a rotating polygonal mirror is frequently used to direct the beam to the grating. A scan mirror oscillating at constant amplitude can have the same function.
In the case of pulse repetition frequencies of about 100 to 1000 pulses per second, rather high rotational velocities of the grating or of the reflecting mirror result. Consequently the time of laser triggering has to be observed with very narrow tolerances, typically about 0.1 microseconds. This accuracy necessitates sophisticated control equipment and involves major technological problems in particular in the case of pulsed CO.sub.2 lasers; these problems may reduce the tuning accuracy and slightly vary the direction of the emitted beam.